Abstract: The polymerization of hexachlorocyclotriphosphazene to poly(dichlorophosphazene) at elevated temperatures is a highly unpredictable process. The rate of polymerization varies over a wide range and is dependent on the history of the trimer and on the method of handling prior to polymerization.It appears that small amounts of water may possibly function as catalytic species for the polymerization. The addition of controlled amounts of water to the system results in a marked increase in reaction rate, and the rigorous exclusion of water from the system inhibits the reaction. The results favor the view that complete exclusion of water from the system may result in a system that cannot be polymerized. Thus, water appears to be a key component for polymerization.